A bearing is a mechanical element which fixes a rotary shaft at a predetermined position and supports self-weight of the shaft and load applied to the shaft to rotate the shaft.
An air foil bearing among bearings is a bearing supporting load when air, which is a fluid with viscosity between foils getting in contact with a rotor or a bearing disc according to high speed rotation of the rotor or a rotary shaft, is introduced and forms pressure.
Moreover, an air foil journal bearing among the air foil bearings is a bearing for supporting a radial-direction load of the rotor, which is a vertical direction to the rotor.
In this instance, as shown in FIG. 1, the air foil journal bearing includes a bearing housing 10, a bump foil 20 mounted along an inner circumferential surface 12 of a hollow 11 of the bearing housing 10, a top foil 30 arranged inside the bump foil 20, and a rotor 40 or a rotary shaft arranged inside the top foil 30, wherein the rotor 40 is rotated in a state that the inner circumferential surface of the top foil 30 and the outer circumferential surface of the rotor 40 are spaced apart from each other. Here, the bump foil 20 and the top foil 30 have bent portions 21 and 31 formed by end portions of a circumferential direction bent outwardly in the radial direction. The bent portions 21 and 31 are inserted into a slot 13 formed in the bearing housing 10 to be combined with the bearing housing 10, so that the bump foil 20 and the top foil 30 do not rotated or are not pushed out and are fixed to the bearing housing 10.
FIG. 2 is a graph showing an interval (h) between the top foil and the rotor and pressure distribution (p) of air according to a location or an angle of the bearing housing when the rotor in FIG. 1 is rotated.
As shown in FIG. 2, when the rotor 40 is rotated, as shown in the drawing, pressure is formed by air existing between the top foil 30 and the rotor 40 so that the rotor 40 is rotated in a state that the rotor 40 is spaced apart from the top foil 30. That is, when the rotor 40 is stopped, a lower side of the rotor 40 is arranged on the top foil 30 by self-weight of the rotor 40 to be supported, and when the rotor is rotated, the rotor 40 is rotated in a floating state. In this instance, a rotational center of the rotor 40 does not coincide with the center of the bearing housing 10, and the rotor 40 rotates in a state that the rotational center of the rotor 40 is one-sided toward one side from a rotational center of the bearing housing 10. In more detail, the rotational center of the rotor 40 is located lower than the center of the bearing housing 10, and when the rotor 40 rotates in the counterclockwise direction, the rotational center of the rotor 40 is formed to be sided to the right.
As described above, when the rotational center of the rotor 40 does not coincide with the center of the bearing housing 10 and the rotor 40 rotates in the one-sided state, if it is used to an air compressor, a rotational accuracy of an impeller combined with one side of the rotor 40 to be rotated with the rotor 40 is deteriorated, so it may deteriorate efficiency and performance of the air compressor.
FIG. 3 is a relation diagram and a graph showing intervals among the bearing housing, the foil and the rotor to which three foils are applied (according to a non-patent literature number 001).
In order to solve the above-mentioned problem, as shown in FIG. 3, when the number of foils (top foils or bump foils) of a bearing is increased, because pressure generated by an inner flow is increased and different movements of three pads may increase rigidity and damping effect, it is technology to use three foils rather than one foil.
That is, three slots are formed in a bearing housing at intervals of 120 degrees and the foils are inserted into the slots to provide mechanical preload in order to enhance high speed stability of the rotor. Furthermore, such technology shows that a gas foil bearing having preload enhances high speed stability of oil-free turbocharger and is less friction loss than a bearing with reduced oil film clearance.
However, such technology can increase rigidity and damping effect through independent movement of the three pads and ascend pressure generated through the inner flow of a joining part. However, the conventional technology is difficult to inspect assembly tolerance and to do mass production if components of the bearing is more than necessary since three foils must be used.
Additionally, In FIG. 1, if a bump is assembled in a protruding state by a worker's mistake when one foil is assembled to the bearing, operation safety of the rotor is deteriorated, and severely, the bump digs into one side of the impeller of the opposite side and causes abrasion of the impeller. In addition, such an unstable movement causes a biased friction of the bearing and deteriorates durability.
As disclosed in Korean Patent No. 10-0749828 including a bearing to which one foil is applied, the bearing can fix a back-and-forth movement of a journal bearing using a stopper and serve as a seal by blocking an air gap between a top foil and a bearing housing. However, the conventional bearing is disadvantageous in cooling since heat generated by friction of a bump cannot get out, and it is difficult to promote mass production due to difficult management of tolerance between the inner diameter of the stopper and the outer diameter of a shaft since the bump is transformed by vibration of the shaft.